Lightning safety

How Lightning Strikes: -

Lightning strikes quick: the entire procedure normally takes under 1/tenth of a second. Ventured pioneers leave a cumulonimbus cloud and a few pioneers move toward the ground. They have numerous branches, yet just 1-2 branches will achieve the ground. Around each 50 meters (yards and meters can be utilized reciprocally) another progression leaves the past stride and heads in a genuinely irregular bearing. On the off chance that a pioneer gets inside around 100m starting from the earliest stage, emphatically charged upward pioneers (or streamers) begin ascending from the nearest grounded objects towards the adversely charged pioneer. This strike separation can fluctuate tenfold. When the descending pioneer is sufficiently close to an upward pioneer, they append to each other and "pioneer a trail" for a critical electric current (an arrival stroke) to shoot from the beginning the cloud. The arrival stroke warms the lightning channel to an amazingly high temperature in a brief timeframe, shaping the noticeable lightning blaze and bringing about thunder as the lightning channel grows quickly. This progression pioneer seek separation idea is critical to comprehend to stay away from upward pioneers and direct strikes. Most ground strikes happen underneath the cumulonimbus cloud, yet numerous still strike past the pole of downpour or past the edge of the cloud. This is essential for lighting wellbeing since it doesn't should rain or even shady overhead for you to be in risk from lightning. Infrequently, a stroke of lightning can move on a level plane and strike some place "all of a sudden" (out of nowhere sky) similarly as 10 miles (16km) away or further. These level strikes are phenomenal and unusual, so they shouldn't influence our choices with one special case: we ought to keep away from the most astounding danger zones if anyplace almost an electrical storm.

Utilizing the 50m pursuit separation of ventured pioneers lightning tends to hit the nearest protest inside that reach toward the end of the last stride. Lightning tends to hit hoisted sharp landscape highlights like peaks. Lightning tends to hit tall trees in open zones, with items twice as high accepting around 4X the strikes. Lightning tends to hit shrubberies in the desert if the hedge is staying up higher than the level ground around it. Lightning hits individuals that are higher up than their prompt environment. Lightning tends to hit a vessel on the water, particularly on the off chance that it has a tall pole. Lightning can even now hit level ground or water, however more haphazardly than it hits raised articles. On the large scale, lightning strikes all the more regularly at higher rises in a dry (mainland) atmosphere like in the Rocky Mountains. Lightning thickness maps demonstrate more lightning at lower rises than on the edges in the Appalachian Mountains, which are more moist (a more sea atmosphere). Lightning thickness maps can demonstrate nearby examples, which give you more particular data. On the neighborhood scale, regardless you have to keep away from higher territory in either atmosphere: that is, maintain a strategic distance from high handles at any elevation. The 50m pioneer seek separation idea ought to help you comprehend why this is imperative.

Lightning frequently hits long electrical transmitters and gets directed along them. Metal wall, electrical cables, telephone lines, handrails, measuring tapes, spans and other long metallic articles can lead streams. Long conductors that are protected starting from the earliest stage can convey more present further from the strike point. High voltage tends to traverse the surface of numerous conductors. A ground strike ordinarily hits an essential item and afterward scatters until it disperses. How it scatters relies on upon numerous variables however the voltage stays higher along better conductors. These conductors may even incorporate wet landscape. Without long conductors and surface circular segments (underneath), information from lightning striking group proposes that a lightning strike is dangerous out to about 10 meters from the strike point, with 1-2 fatalities and many wounds. A few people infrequently get harmed 50-100' from a strike. This is generally equal to the murder range and harm sweep of a hand explosive.

The Science of Lightning: -

By definition, all electrical storms contain lightning. Lightning can strike the ground or ground-based articles, inside the electrical storm cloud, starting with one rainstorm then onto the next or into the air. Storms are destined to create on warm spring or summer days, yet they can happen at whatever time of the year.

The development of a thunderstorm: -

Pockets of air ascend into the environment, either constrained by a front or because of warming of the earth on a sunny day. When this air achieves a specific level in the climate, cumulus mists begin to frame. Kept warming of the damp air can bring about these mists to become vertically upward in the environment, into "towering cumulus" mists. These mists might be the principal sign of a creating rainstorm, or cumulonimbus cloud.

How lightning forms: -

Lightning is created due to the blend and crashes of ice precious stones high in the electrical storm with raindrops and hailstones in the lower parts of the tempest. The lighter ice precious stones turn out to be decidedly charged and are conveyed into the most elevated parts of the cloud. Heavier hail and rain assemble a negative charge and fall toward the lower part of the cloud. The world's surface ordinarily has a slight negative charge. Be that as it may, as the negative energizes work in the lower part of the cloud, the ground underneath it and encompassing areas turn out to be emphatically charged. At first, the air goes about as an encasing between these varying charges. In any case, when the electrical potential between the two charges turns out to be excessively incredible, there is a release of power known as LIGHTNING.

Lightning and thunder: -

Lightning is the mammoth sparkle of power that happens inside the environment. As it goes through the air, the one-inch distance across or less electrical jolt quickly warms the air to a temperature of 50,000 degrees Fahrenheit, which is more sweltering than the surface of the sun. The air grows quickly because of the warming, then rapidly contracts as it cools back to its typical temperature. This makes a shockwave that we hear as THUNDER.

Heat lightning: -

Heat lightning is essentially lightning from a removed electrical storm that is too far away for the resultant thunder to be listened. As a rule, the light you watch is being reflected off of mists close to the skyline many miles away. Watch out for the tempest however, since it might be traveled toward you.

Lightning Injury Mechanisms: How Can Lightning Hurt Us?

Lightning tosses a gathering of destructive and harmful dangers our way. These impacts happen in the same couple of milliseconds, yet none of the dangers wait after every strike. These components are organized underneath by request of how regularly they add to fatalities. Displaying them all together backings showing their relative significance.

Ground Current/ Step Voltage: -

Ground current happens with every strike and causes generally 50% of all lightning wounds. Ground streams are driven by the tremendous potential contrasts that show up in the earth close to the ground strike point. Ground current is additionally alluded to as earth potential ascent (EPR). EPR is an all the more in fact exact term however ground current might be less demanding for nonexperts to grasp. High voltage isn't the principle issue: what is important is whether one a player in your body gets in touch with one voltage and another piece of your body contacts an alternate voltage: the distinction in voltage is the thing that drives current through your body. Voltage is the potential for current to travel through you, which is the reason it is likewise just called potential. The potential distinction can drive an electric current up one leg and down the other of a man or creature, with the measure of current relying upon the potential contrast, the separation between the feet, and the introduction of the contrast between the feet – in this way the expression "step" voltage.

Surface circular segments are connected with ground streams and are all the more appropriately called ground surface bends. High present surface curves seem, by all accounts, to be connected with some part of all cloud-to-ground releases, amid the arrival stroke. They show up in photos as brilliant bends of light emanating from a strike point like spokes of a wheel, noticeable all around simply over the ground's surface These long, hot flat streams have been measured up to 20 meters long and can be longer. On the off chance that you are in the way of a surface curve you are liable to lead a portion of the surface circular segment current through or over your body. Commonplace lightning-to-ground strikes infuse approximately 20,000 amps into the Earth: subsequent to the Earth opposes electrical stream, substantial potential contrasts will show up in the ground all around the strike point. How far the present streams change generally since strike current and ground conductance effortlessly shift by requests of size. In any case, the nearer you are to the immediate strike, the more grounded the ground current. In the event that you are remaining with your legs isolated, in the event that you are on all fours, on the off chance that you are in an inclined position on the ground, or in the event that you are touching a long metallic item, you boost your introduction to potential contrasts that emerge from ground streams. The term utilized for the voltage contrast one stage (1m) separated is step voltage. The potential contrast that shows up between your legs or over your inclined body can drive critical streams through and over your body. You can minimize your presentation to ground streams by keeping your feet near one another, particularly evading an inclined position. These activities can minimize the measure of ground current experiencing your body, however most specialists think these endeavors are debatable contrasted with getting to a more secure area. We should be watchful that we don't give individuals a misguided sensation that all is well and good by getting in the lightning position. Ground current adds to 40-half of lightning fatalities so this is the essential component we ought to consider when decreasing lightning dangers.

Side flash: -

At the point when lightning hits a tree or other tall question, the fundamental current takes after the tree trunk to the ground, however some current may curve over the air to an easiest course of action (like individuals) that can lead the current to the ground. This is like surface circular segments, however off the ground. Side glimmer is more critical in tall items with higher resistance than with low resistance so it happens more altogether with trees than with towers. Since side glimmer exudes from trees or other tall articles struck by lightning, never look for asylum close to a tree, other tall item, or tall vertical surface. Side glimmer adds to 20-30% of lightning fatalities. Side blaze is one reason that the "cone of assurance" is a myth.

Upward streamer currents (upward leaders): -

Quick high current heartbeats are dispatched from the highest points of numerous lifted articles close to each descending pioneer as it methodologies the ground. These are dispatched because of the immensely high electric field that exists, quickly, under every tip of the ventured pioneer. Since the tips of a few or numerous pioneer's may approach the ground at about the same time, you don't need to be extremely close to the real ground strike point to be required in a streamer current. Streamer ebbs and flows, while much littler than the arrival stroke current, are still sufficiently huge to bring about harm or passing to people. You stifle the propensity to dispatch streamer ebbs and flows from your individual by squatting into a tight ball as near the ground as could be allowed. You maintain a strategic distance from this probability by keeping away from high areas. Upward streamers add to 10-15% of lightning fatalities.

Touch voltage: -

Touch or contact voltage happens when we touch a jolted article like a wall or a corded phone. This doesn't have the current of an immediate strike yet can be deadly. An Internet look effortlessly discovers pictures of gatherings of dead creatures that were touching wall when lightning struck the wire. Contact wounds add to 15-25% of lightning fatalities.

Direct strike: -

This implies the ventured pioneer associated with a streamer leaving your body, then the arrival stroke disregarded through you or your body's surface. The arrival stroke is the most critical electrical occasion of a lightning strike and has a run of the mill current of 20,000 amps. You significantly lessen the odds of accepting an immediate strike by being inside a considerable building or a metal-topped vehicle. In the backcountry you ought to maintain a strategic distance from high places and open ground to lessening danger of an immediate strike. Direct strikes include around 3-5% of lightning fatalities so we ought to principally center our endeavors on keeping away from ground present, side blaze, and different instruments of damage. Shockingly, past lightning instruction harped on direct strikes as the essential system of harm, so individuals set down to get lower, subsequently expanding the odds of getting harmed by the essential lightning damage component which is ground current.

Types of Lightning: -

Intra-Cloud: The most common type of lightning. It happens completely inside the cloud, jumping between different charge regions in the cloud. Intra-cloud lightning is sometimes called sheet lightning because it lights up the sky with a 'sheet' of light.

Cloud to Cloud: Lightning that occurs between two or more separate clouds.

Cloud to Ground: Lightning that occurs between the cloud and the ground.

Cloud to Air: Lightning that occurs when the air around a positively charged cloud top reaches out to the negatively charged air around it.

Bolt from the blue: A positive lightning bolt which originates within the updraft of the storm, typically 2/3rds of the way up, travels horizontally for many miles, then strikes the ground.

Anvil Lightning: A positive lightning bolt which develops in the anvil, or top of the thunderstorm cloud, and travels generally straight down to strike the ground.

Heat Lightning: Lightning from a thunderstorm that is too far away to be heard.

Lightning Facts: -

• Lightning causes an average of 80 deaths and 300 injuries each year.
• The energy from one lightning flash could light a 100watt light bulb for more than 3 months.
• Only about 10% of people that are struck are actually killed. 90% survive, but nearly 25% of these survivors suffer long term psychological or physiological trauma.
• 92% of lightning injuries and deaths occur between May and September.
• 45% of the deaths and 80% of injuries occurred in these months between 10am and 7pm.
• Rubber-soled shoes & rubber tires provide NO protection from lightning.
• Lightning can strike more than 10 miles away from any rainfall. More than 50% of lightning deaths occur AFTER the storm has passed.

  EFFECTS OF LIGHTNING STROKE: -

  Electrical Effects: - As the current is discharged through the resistance of the earth electrode of the lightning protective system, it produces a resistive voltage drop which may momentarily raise the potential of the protective system to a high value relative to true earth. It may also produce around the earth electrodes a high potential gradient dangerous to persons and animals. In the same general manner. the inductance of the protective system must also be considered because of the steep leading edge of the lightning pulse. The resulting voltage drop in the protective system is. therefore, the combination of the resistive and inductive voltage components.

  Side flashing: - The point of strike on the protective system may be raised to a high potential with respect to adjacent metal. There is. therefore, a risk of flashover from the protective system to, any other metal on or in the structure. If such flashover occurs, part of the lightning current is discharged through internal installations, such as pipes and wiring, and so this flashover constitutes a risk to the occupants and fabric of the structure.

  Thermal: - As far as it affects lightning protection, the effects of a lightning discharge is confined to the temperature rise of the conductor through which the current passes. Although the current is high, its duration is short, and the thermal effect on the protective system is usually negligible. (This ignores the fusing or welding effects on damaged conductors or choose which were not adequate in the initial installation.") In general, the cross-sectional area of a lightning conductor is chosen primarily to satisfy the requirements of mechanical strength, which means that it is large enough to keep the rise in temperature to 1°C. For example, with a copper conductor of 50 mm2 cross section, a severe stroke of 100 kA with a duration of 100 micro second dissipates less than 400 J/per metre of conductor resulting in a temperature rise of about 1°C. The substitution of steel for copper results in a rise of less than 1O0C.

  Mechanical Effects: - Where a high current is discharged along parallel conductors at close proximity, or along a single conductor with sharp bends, considerable mechanical forces are produced. Secure mechanical fittings arc, therefore, essential. A different mechanical effect exerted by a lightning Bash is due to the sudden rise in air temperature to 30 000 K and the resulting explosive expansion of the adjacent air in the channel along which the charge is propagated. This is because, when the conductivity of the metal is replaced by that of an arc path, the energy increases about one hundredfold. A peak power of about 100 MW/m can be attained in the return. stroke and the shock wave close to this stroke readily dislodges tiles from a roof. Similarly, with a secondary flash inside the building, the shock wave can result in damage to the building fabric.

  Scheme design criteria: -

  The ideal lightning protection for a structure and its connected services would be to enclose the structure within an earthed and perfectly conducting metallic shield (box), and in addition provide adequate bonding of any connected services at the entrance point into the shield. This in essence would prevent the penetration of the lightning current and the induced electromagnetic field into the structure. However, in practice it is not possible or indeed cost effective to go to such lengths. This standard thus sets out a defined set of lightning current parameters where protection measures, adopted in accordance with its recommendations, will reduce any damage and consequential loss as a result of a lightning strike. This reduction in damage and consequential loss is valid provided the lightning strike parameters fall within defined limits, established as Lightning Protection Levels (LPL).

  External Lightning Protection System (LPS) design considerations: -

  The lightning protection designer must initially consider the thermal and explosive effects caused at the point of a lightning strike and the consequences to the structure under consideration. Depending upon the consequences the designer may choose either of the following types of external LPS:

  • Isolated
  • Non-isolated

  An Isolated LPS is typically chosen when the structure is constructed of combustible materials or presents a risk of explosion. Conversely a non-isolated system may be fitted where no such danger exists.

  An external LPS consists of:

  • Air termination system
  • Down conductor system
  • Earth termination system

  These individual elements of an LPS should be connected together using appropriate lightning protection components (LPC). This will ensure that in the event of a lightning current discharge to the structure, the correct design and choice of components will minimize any potential damage.

  Air termination system: -

  The role of an air termination system is to capture the lightning discharge current and dissipate it harmlessly to earth via the down conductor and earth termination system. Therefore, it is vitally important to use a correctly designed air termination system.

  • Air rods (or finials) whether they are free standing masts or linked with conductors to form a mesh on the roof
  • Catenary (or suspended) conductors, whether they are supported by free standing masts or linked with conductors to form a mesh on the roof
  • Meshed conductor network that may lie in direct contact with the roof or be suspended above it (in the event that it is of paramount importance that the roof is not exposed to a direct lightning discharge)

  The standard makes it quite clear that all types of air termination systems that are used shall meet the positioning requirements laid down in the body of the standard. It highlights that the air termination components should be installed on corners, exposed points and edges of the structure. The three basic methods recommended for determining the position of the air termination systems are:

  • The rolling sphere method
  • The protective angle method
  • The mesh method

  The rolling sphere method: -

  The rolling sphere method is a simple means of identifying areas of a structure that need protection, taking into account the possibility of side strikes to the structure. The basic concept of applying the rolling sphere to a structure is illustrated in below Figure

  
• Application of the rolling sphere method
• 
• Maximum values of rolling sphere radius corresponding to the Class of LPS.

  This method is suitable for defining zones of protection for all types of structures, particularly those of complex geometry.

  The protective angle method: -

  The protective angle method is a mathematical simplification of the rolling sphere method. The protective angle (a) is the angle created between the tip (A) of the vertical rod and a line projected down to the surface on which the rod sits. The protective angle afforded by an air rod is clearly a three dimensional concept whereby the rod is assigned a cone of protection by sweeping the line AC at the angle of protection a full 360o around the air rod. The protective angle differs with varying height of the air rod and class of LPS. 

  

  The protective angle method for a single air rod

  Varying the protection angle is a change to the simple 45o zone of protection afforded in most cases. Furthermore, the new standard uses the height of the air termination system above the reference plane, whether that be ground or roof level.

  

  Note 1: -   Not applicable beyond the values marked with Only rolling sphere and mesh methods apply in these cases.

  Note 2: - h is the height of air-termination above the reference plane of the area to be protected

  Note 3: - The angle will not change for values of h below 2m

  Determination of the protective angle

  The protective angle method is suitable for simple shaped buildings. However, this method is only valid up to a height equal to the rolling sphere radius of the appropriate LPL.

  

  Effect of the height of the reference plane on the protection angle

  The mesh method: -

  four different air termination mesh sizes are defined and correspond to the relevant class of LPS. This is the method that was most commonly used.

  

  Maximum values of mesh size corresponding to the Class of LPS

  This method is suitable where plain surfaces require protection if the following conditions are met:

  • Air termination conductors must be positioned at roof edges, on roof overhangs and on the ridges of roof with a pitch in excess of 1 in 10 (5.7o)
  • No metal installation protrudes above the air termination system

  Modern research on lightning inflicted damage has shown that the edges and corners of roofs are most susceptible to damage. So on all structures particularly with flat roofs, perimeter conductors should be installed as close to the outer edges of the roof as is practicable.

  

  Concealed air termination network

  The current standard permits the use of conductors (whether they be fortuitous metalwork or dedicated LP conductors) under the roof. Vertical air rods (finials) or strike plates should be mounted above the roof and connected to the conductor system beneath. The air rods should be spaced not more than 10 m apart and if strike plates are used as an alternative, these should be strategically placed over the roof area not more than 5 m apart.

  Non-conventional air termination systems: -

  A lot of technical (and commercial) debate has raged over the years regarding the validity of the claims made by the proponents of such systems. This topic was discussed extensively within the technical working groups. The outcome was to remain with the information housed within this standard. The volume or zone of protection afforded by the air termination system (e.g. air rod) shall be determined only by the real physical dimension of the air termination system. Typically, if the air rod is 5 m tall then the only claim for the zone of protection afforded by this air rod would be based on 5 m and the relevant class of LPS and not any enhanced dimension claimed by some non- conventional air rods.

  Estimation of Exposure Risk: -

  'The probability of a structure or building being struck by lightning in anyone year is the product of the 'lightning flash density' and the 'effective collection area' of the structure. The lightning flash density, N, is the number of (flashes to ground) per km2 per year.

  

  The effective collection area of a structure is the area on the plan of the structure extended in all directions to take account of its height. The edge of the effective collection area is displaced from the edge of the structure by an amount equal to the height of the structure at that point. Hence. for a simple rectangular building of length L, width W and height H metres, the collection area has length (L + 2H) metres and width (W +2H) metres with four rounded corners formed by quarter circles of radius H metres. This gives a collection area, Ao (in m2):

  Ao = (L X W) + 2 ( I., X H) + 2 ( W X H) +π H3

  The probable number of strikes (risk) to the structure per year is

  P = Ao X N1 X 10-8

  It must first be decided whether this risk P is acceptable or whether some measure of protection is thought necessary. For the purposes of this Cod e, the acceptable risk figure has been taken as 10-5, that is, 1 in 100 000 per year.

  Conclusion: -

  You can tell how close you are to a lightning strike by counting the seconds between seeing the flash and hearing the thunder. For every five seconds you count, the lightning is one mile away. If you see a flash and instantly hear the thunder, the lightning strike is very close. Take shelter immediately. A hand bearing compass can be used to determine if you are in the path of a storm. By observing the storm’s ground flashes and using a series of bearings, you can plot the approach of a thunderstorm. If the bearing of the flashes doesn’t change, the average storm is heading toward you. It is time to alter your course.